Plutonium Oxidation States in Complex Molecular Solids

Oxidation state is a key chemical quantity that allows the understanding and prediction of the majority of chemical reactions; however, the main deficiency using the formal oxidation state comes from the materials containing multivalent metals. Among them, the most complicated element plutonium (Pu) is an outstanding instance. Here, we calculate the orbital occupation numbers under the frameworks of first-principles density functional theory (DFT) + U methods to quantitatively determine the Pu-oxidation state of the recently reported complex molecular solids: [K(crypt)]Pu[C₅H₃(SiMe₃)₂]₃, Pu₃(DPA)₅(H₂O)₂, and Pu₃(DPA)₆H. The results show that the oxidation state of Pu is an extremely low Pu²⁺ in the former and a mixed Pu³⁺/Pu⁴⁺ in the latter two compounds, which is consistent with the experimental identifications. The steric effects and the environmentally sensitive localization → delocalization transition of Pu 5f electrons can rationally elucidate the formation of the unusual oxidation states. Such atomic-resolution quantitative determination of the unusual Pu oxidation states in the complex molecular solids offers an alternative for the further exploration of peculiar Pu solid-state materials.